For over 100 years the fire and emergency alarm equipment industries have used a “telegraph” signaling scheme to give the public a means for alerting responders to emergency situations. These are largely electro-mechanical devices. These sending mechanisms have been made by companies such as Gamewell, a division of Honeywell, Faraday, and the Peerless Company. During their many years of availability, these emergency signaling systems have become a familiar site in the form of fire alarm boxes on street corners, emergency alarm boxes in subway tunnels, etc. Unfortunately, because they are largely mechanical in nature, the sending mechanisms themselves require service. This is a significant expense, both in material and labor, especially since the sending units are used in large quantities.
Electronic replacements have been slow to replace these units because they typically require local power, which is not a convenient and reliable option. Augmenting local power reliability with a local rechargeable battery is also an unfavorable option because this battery will require replacement every three to five years. Alternately, bringing reliable power to each unit location from the master alarm panel would require the addition of conductors, which is also viewed as prohibitive by potential end users. A solid state electronic replacement device that could operate entirely from the 100 milliamp DC loop current associated with the present system would likely fill a major void in the market. Such a system is the subject of the present invention.
In switching to a solid state replacement for existing electro-mechanical devices consideration must be given to at least the following:
a) the electronic replacement device must be electrically and operationally compatible with existing mechanisms, as well as additional electronic units within the loop.
b) The device must be powered entirely from the 100 milliamp DC loop current. It must not require any local power nor employ any local battery, regardless of type.
c) Protection from voltage and current surges must be incorporated.
d) The device must operate on loop current applied in either direction.
e) The device circuitry must be completely isolated from local ground.
f) Any switch contacts carrying the loop current must be field serviceable.
g) When the device is initially activated by the user, it must not immediately open the current loop. Instead, it must wait and watch the loop for activity (circuit openings) occurring at another device on the loop. This wait time must be approximately the time between ID repetitions, to truly ensure that another unit is not running.
h) If activity is present, the device must wait to send its ID after activity elsewhere ceases.
i) Once the replacement device begins to send its ID, it must ignore any further interruptions in the loop, as long as they are brief enough to maintain its operation.
j) For the purposes of “non-interference” sensing, a loop current greater than a first predetermined amount, for example, approximately 70 mA, must be considered closed, while a loop current less than a second predetermined amount, for example, approximately 17 mA, must be considered open. This approximates the hysteresis of the relays that are typically employed to monitor the current loops.
k) Once activated, a single device emulating a closed loop must result in a current greater than yet another predetermined amount, for example, 90 mA. A single device emulating an open loop must result in a current less than a further predetermined amount, for example, 2 mA. When switching from closed loop to open loop emulation, a brief moment of zero loop current is desirable to ensure release of the loop monitoring relay (for example K1 in FIG. 1), regardless of its coil sensitivity.
l) The existing mechanisms require a replacement cam to change their ID. The setting or altering of the ID of the electronic replacement must be configurable in the field with only a few hand tools. Access to changing the ID must require at least one hand tool, precluding access to the public.
m) Once a unit is initially activated, repeated depressions of the initiating operator must not affect its operation until its transmission cycle is complete.
n) Upon completing its transmission, the unit must return the loop to its closed state and be ready for repeated use without requiring any maintenance such as winding, charging, manual reset, manual opening of the loop, etc.
o) The replacement device must provide visual feedback, such as an LED, to indicate to the user that his signal will be sent. For example, if loop current was not present when the user depressed the operator or if the operator mechanically malfunctioned, this LED must not illuminate.
p) Multiple units must be stackable and must activate from a single operator. This allows a single physical location to be part of multiple current loops while fully preserving each unit's individual features. This characteristic is useful in subway tunnels, where the loop monitoring equipment also switches off power to the third rail and where a single location must affect multiple sections of third rail.
q) The visual indication on the top device of a multiple device stack must illuminate only if all devices in the stack have confirmed their loop current and latched the depression of the operator.
r) Each device in a multiple device stack must latch the confirmation from the device below it, so that confirmations do not need to occur simultaneously to produce visual confirmation from the device at the top of the stack.
s) Confirmations must be passed up within a stack using optoelectronics. Electrical connection between loops is not permitted. Inductive coupling of a signal is also undesirable due to its susceptibility to electromagnetic interference.
t) The electronic replacement device must be compact and designed to fit in the existing outer housings originally designed to house their mechanical grandfathers.
u) The device must be no thicker than 0.75″ in order to stack up to five high in the original outer housings.